Let's see (been awhile)
1. remove the black square cover off the AFM
-run a razor blade around the outside to cut the seal
-pry out the square cover
-remove all the old silicone
2. you should be able to see the electronics of the AFM and a black gear in the middle
3. locate the W- shaped spring in the lower left hand corner held down by one screw
4. mark a reference location on the gear and the housing in some permanent manner (i.e. paint, engraving, etc.) (IMPORTANT!)
5. now holding down on the gear loosen the screw in the lower left hand corner a few turns
6. Rotate the gear anywhere from 1 to 3+ teeth counter-clockwise and retighten the screw
7. reinstall the black square cover and apply silicone around the perimeter like Toyota did (electronics and water don't mix)
Basically what you're doing is changing the spring tension on the flapper inside the airstream. By going counter-clockwise will lessen the tension, so that at a given RPM/airflow the flapper will open more. Techically what happen is the computer thinks more air is flowing than what there is, so this richens the fuel mixture. The opposite is true if you go the other way (more tension/leaner mixture). You should probably try 2 teeth to begin with. You can get by taping the cover on and test drive it. Keep rotating an extra tooth at a time till you get to where the engine will feel like its bogging down, and go back a tooth or two. And then go ahead and do step 7. Note this won't help your gas mileage any.
Couple of links for the same thing
http://www.geocities.com/motorCity/7177/toyafmmod.html
http://www.celicas.org/celica_features.htm
(click peformance mods, airflow meter)
Aaron I
